Solid electrolyte-based gas sensors, by which the concentration of individual gas components can be determined electrochemically, have been used for some time in the analysis of exhaust gases of internal combustion engines. The mode of operation of some of these gas sensors is based on the fact that the oxygen concentration within one sensor element integrated into the gas sensor is set at a constantly low value. This is accomplished by an electrochemical pumping process in which oxygen transfer takes place between the electrodes of an electrochemical pumping cell. One of the electrodes of the pumping cell is located on the sensor element's outer surface that is exposed to the exhaust gas. Although this outer pumping electrode is frequently provided with a porous protective coating, it is nonetheless exposed to the corrosive effects of the hot combustion exhaust gases.
U.S. Pat. No. 4,902,400 describes a sensor element in which the oxygen concentration within the sensor element is set at a constant value, without the sensor element having an outer pumping electrode attached to the outside of the sensor element. Instead, the sensor element includes two additional solid electrolyte layers and an additional reference gas duct that is in contact with the ambient atmosphere through an opening and in which the outer pumping electrode is located. This sensor element design is complex, however, and therefore cost-intensive.
The object of the present invention is to provide a sensor element whose outer pumping electrode is not directly exposed to the gas mixture under analysis and which at the same time has as simple a design as possible.